In conventional automotive internal combustion engines, efficiency of intake to the combustion chamber has been low especially when the passage is choked by the throttle valve. Further, ignitability and combustibility drop during idling and light-load operation when the air-fuel mixture flows into the combustion chamber at a low rate.
Generally, such deterioration in ignitability and combustibility is solved by supplying rich mixtures, i.e., at low air-fuel ratio, that burns well. But this solution entails an increase in fuel cost and such noxious unburned emissions as hydrocarbons and carbon monoxide.
Also, especially with the object of decreasing poisonous nitrogen-oxide emissions, it has recently been proposed to burn such lean mixtures whose air-fuel ratios are substantially higher than the stoichiometric value, and to burn air-fuel mixtures with part of the exhaust gas recirculated from the exhaust system of the engine. In both cases, however, the mixture ignitability and combustibility drop, which in turn impairs the car's driveability and fuel economy.